Heat-treating magnesium-alloy die castings



Patented Sept. 21, 1948 HEAT-TREATING MAGNESIUM-ALLOY DIE CASTINGSRobert C. Cornell, Jackson, Mich, assignor .to The Dow Chemical Company,Midland, Mich, ,a corporation of Delaware No Drawing. ApplicationAugust1, 1946, SerialNo. 687,703

This invention relates to the heat-treatment of .760 to .790 F. from amagnesium-base alloy pressure die-castings.

Magnesium-alloy sand and permanent-mold castings are conventionallyheat-treated to develop improved properties. However, the application ofsimilar heat-treatment to pressure diecastings of the same metal hasbeen largely unsuccessful. Thus, While such die-castings are sometimesgiven a, conventional solution-heattreatment to relieve stresses or toimprove ductility, there has always been an accompanying seriousdecrease in yield strength. Aging treatments likewise decrease thestrength of the casting and also lower the ductility, For these reasons,magnesium-base alloy die-castings have almost always been used in theas-cast condition.

It has now been found, however, that both the strength and the ductilityof magnesium-base a1- loy die-castings may be materially improved bysolution-heat-treatment, provided the castings are first subjected toartificial aging. This discovery forms the basis of the presentinvention.

In practice of the invention, the pressure diecast article of asolution-heat-treatable magnesium-base alloy, in the as-cast condition,is first heated at an aging temperature for a time sufiicent to producesubstantial precipitation of a major additional phase in the alloy. Inthe case of the common magnesium die-casting alloys containing from 4 to12 per cent by weight of alumif num as the major alloying element,temperatures in the range of 325 to 700 F. are preferred, with times ofat least several hours being usually required. For example, amagnesium-base alloy containing 8 to 10 per cent of aluminum, 0.4 to 1.0per cent Zinc, and 0.1 to 0.5 per cent manganese as the major alloyingelements is best aged at from about 325 to about 550 F. for a period offrom about to about 20 hours, the longer times being required at thlower temperatures.

Following the aging procedure, the article is then given asolution-heat-treatment for a time sufiicient to effect a partial orpreferably a nearly complete solution of the precipitated additionalphase. Temperatures conventional for solutionheat-treatingmagnesium-alloys are employed, the times of heating being slightly lessthan those ordinarily used with sand-cast magnesium articles. Forexample, with the 4 to 12 per cent aluminum-containing alloy, atemperature of at least 715 F. for at least several minutes ispreferred. In the case of the magnesium-aluminumzinc-manganese alloymentioned above, the solution-heat-treatment is best carried out at from4 Claims. (Cl. 148-21.90)

time of from about 15 minutes to :about :2 hours.

Following the solution-heat-treatment, the article is cooled toroomtemperature and is ready for use.

' Metallographic studies give .a possible explana- .tion of theeffectiveness of the heat-treatment according to the invention. Thus,when die-cast magnesium alloy articles are subjected tosolution-heat-treatment without preliminary aging, a non-germinative butregular grain growth oc ours, with resultant weakening of the metal.However, when the die-cast article is first given a precipitation oraging heat-treatment according to the invention and is thensolution-heattreated, grain growth during the latter treatment isminimized or even inhibited and the strength and ductility of the metalare improved. This inhibiting effect persists throughoutsolutionheat-treatment, even well after the phase precipitated duringthe aging has been dissolved. The precise cause of the inhibitory effectinduced by the aging is not clear but presumably involves some changesin the energy relations in the crytal lattice of the metal. The effectis limited. to pressure-die-cast articles in which a high degree ofsupersaturation exists due to the drastic chilling of the metal, and isnot observed in sand and permanent-mold castings of the samemagnesium-base alloys.

Since unsound die-castings of magnesium-alloys tend to blister duringheat-treatment, it is important, in practicing the invention, to controlthe initial die-casting operation, in known manner, to produce onlysound metal.

The following example will illustrate the invention.

Example A magnesium-base alloy containing 9.0 per cent by weight ofaluminum, 0.6 per cent zinc, and 0.2 per cent manganese as the majoralloying elements was pressure-die-cast into test bars in a standardcold-chamber casting machine. In the as-cast condition, the metalexhibited the following properties: tensile strength, 35,000 p. s. 1.,yield strength, 22,000 p. s. i., and elongation in 2 inches, 4.0 percent.

The test bars were then subjected to an aging treatment at 500 F. for 18hours, after which they were given a solution-heat-treatment at 780 for1.0 hour. The metal then had the properties: tensile strength, 38,000 p,s. i., yield strength, 18,000 p. s. i., and elongation in 2 inches, 10per cent.

What is claimed is:

1. A heat-treatment for pressure die-cast articles formed. of asolution-heat-treatable magnesium-base alloy which consists in heatingthe as-cast article at an aging temperature for a time sufficient toeffect substantial precipitation, and then subjecting the article tosolutionheat-treatment.

2. A heat-treatment for pressure die-cast articles formed of asolution-heat-treatable magnesium-base alloy containing from 4 to 12 percent by weight of aluminum as the major alloying element which consistsin heating the as-c'ast article at an aging temperature in the range of325 to 700 F. for a time sufiicient to eiTect substantial precipitation,and then subjecting the article to solution-heat-treatment at atemperature of at least 715 F.

3. A heat-treatment for pressure die-cast articies'f'or'med of amagnesium-base alloy containing 8 to 10 per cent by weightor aluminum,0.4 to 1.0 per cent zinc, and 0.1 to 0.5 per cent manganese as the majoralloying elements which consists in heating the as-cast article at anaging temperature of from about 325 to about 550 F. fora period of fromabout to about 20 hon-rs to efiect substantial precipitation, and thensubjecting the article to a solution-heat-treatment at a temperature offrom about 760 to 790 F. for a REFERENCES CITED The following referencesare of record in the file of this patent:

Number UNITED STATES PATENTS Name Date Busk Nov. 21, 1944 OTHERREFERENCES Transactions A. I. M. E., vol. 161, page3l1.

Published in 1945 by the Institute of Mining and MetallurgicalEngineers, New York, New York.

